Paper curl is a common problem in the field of printing and more particularly in the field of electrostatic printing. Often, attempts are made to determine a direction of curl of supplied paper, and the paper is loaded into the supply bins of a printer or copier in a particular direction in accordance with the direction of curl. The printing process itself imparts effects on the paper which can result in curl. Finally, the feeders of a paper handling machine, and particularly the output feeders, are designed to counteract a tendency for paper curling. Since there is a variation in the quality and initial curl of the paper, it is not always possible to discharge paper in a manner which eliminates curling. If the output of a paper handling machine is used to create a stack of paper, it is desired that the stack be in registration, meaning that the ends of the sheets align. In order to accomplish this, the sheets are caused to rest against an end stop, and this end stop defines a registration position of the sheets. This creates two problems with respect to curl: (1) if the paper is curled, the sheets tend to rest high in the bin at the end stop; (2) the movement of the sheets downwardly against the end stop itself encourages curl as a result of friction of the sheets against the end stop.
Paper stack quality is a critical parameter for high capacity output devices on laser printers and copiers. Paper curl is a significant problem when stacking paper coming out of a laser printer. Reasons for this behavior are humidity, temperature during the toner fusing process, toner distribution on the printed paper, composition and weight of the paper, printed side, ejection speed of the paper when leaving the printer, shape of the bin on which the paper is being stacked, etc. Paper curl can be positive curl, as shown in FIG. 1, and negative curl, as shown in FIG. 2. Essentially, positive and negative curl are a function of which way the paper lies; that is, negative curl becomes positive curl if the sheets are tuned over, and vice-versa.
Of the two types of curl previously mentioned, the one that can cause more problems to the device is positive curl. One of the problems that this type of curl can cause is that it can obstruct the paper path as depicted in FIG. 3. Another problem is that the trailing edge of the last stacked paper can touch the eject rollers, allowing it to be transported back to the device, as depicted in FIGS. 4a and 4b. This can cause a paper jam.
Paper curl is normally corrected using decurlers or retainers. Decurlers work by making the paper go through a series of rollers as shown in FIG. 5, instead of using a straight path. This creates a buckle in the opposite direction of the curl; that is, increasing the positive or the negative curl. Some of the disadvantages of this approach are:
Decurlers do not work well on all kinds of paper. Since the curl on the paper can be either positive or negative, and the decurlers work by increasing the positive or the negative curl (but not both), it may be increasing the curl on the paper. Decurlers can cause paper jams.
Retainers on the other hand, are long and flexible fingers mounted on a shaft right above the trailing edge of the stack, as shown in FIG. 6. When the paper leaves the eject rollers, the paper flies above the retainers and then the retainers rotate. On rotation, the retainers grab the last ejected page and push it back against the wall (registering). This also reduces the curl of the paper. One disadvantage of this approach is that it adds more components to the device.